1. Field
This invention relates to electronic ballasts, and more particularly for single stage ballasts and TRIAC dimmable high frequency electrodeless lamps.
2. Description of Related Art
Phase controlled TRIAC dimmers are commonly used for dimming incandescent lamps. TRIAC is a bidirectional gate controlled switch that may be incorporated in a wall dimmer. A typical dimmer circuit with an incandescent lamp is shown in FIG. 1, where the TRIAC turns “on” every half of AC period. The turn “on” angle is determined by the position of the dimmer potentiometer and can vary in range from 0 to 180 degrees. Typically the lighting dimmer is combined with a wall switch. An incandescent lamp is an ideal load for a TRIAC. It provides a sufficient latching and holding current for stable turn “on” state. The TRIAC returns to its “off” state position at the AC voltage zero crossing. But wall dimmers are not capable of dimming a regular single stage ballast. These ballasts are distinguished by front-end power supplies having a rectifier bridge with an electrolytic storage capacitor. Since conduction angle of bridge rectifier is very short and holding current is not provided during the rest of the period, the TRIAC operation becomes unstable and causes lamp flickering.
Besides holding current, the TRIAC should be provided with latching current, that is a sufficient turn “on” current lasting at least 20-30 usec for latching TRIAC internal structure in stable “on” state. A ballast circuit may have an RC series circuit connected across the ballast AC terminals to accommodate the TRIAC. But steady power losses in the resistor could be significant. Other references have similar principles of operation, such as based on drawing high frequency power from the bridge rectifier. Since this power is taken from the output of the ballast the power to the lamp should be lower. The power used to support continuous rectifier bridge current should be significant to provide compatibility with actual lighting dimmers in the field.
Other previous work discloses a TRIAC dimmable electrodeless lamp without an electrolytic storage capacitor. In this case the ballast inverter input current is actually a holding current of the TRIAC and is high enough to accommodate any dimmer. The lamp ballast is built as self-oscillating inverter operating at 2.5 MHz. An example block diagram of a dimmable ballast is shown in FIG. 2. It comprises an EMI filter F connected in series with AC terminals, a Rectifier Bridge providing high ripple DC voltage to power a DC/AC resonant inverter, and a Resonant Tank loaded preferably by inductively coupled Lamp. The ballast Inverter is preferably self-oscillating Inverter operating in high frequency range (2.5-3.0 MHz). A TRIAC dimmer is connected in front of the ballast providing a cut-off phase control of input AC voltage.
Related art teaches operation from a rectified AC line live voltage that bounces from almost zero volts to about 160-170V peak. A self-oscillating inverter may start at some instant DC bus voltage, such as between 80V and 160V, but it will stop oscillating at lower voltage (usually in a range between 20V and 30V). FIG. 3 illustrates a related art dimming method where Vm 302 is a voltage waveform after the TRIAC dimmer. This voltage is rectified and applied to the input of the inverter. Without an electrolytic storage capacitor, the ballast inverter (not shown in FIG. 3) stops its operation during TRIAC “off” intervals. Accordingly, electrical discharge in the lamp burner stops and starts, such as illustrated in lamp current ILAMP 304 in FIG. 3.
Since the recombination time in lamp gas is much shorter than the TRIAC's “off” time the lamp restarts every half period with high starting voltage and power as at regular starting. For an electrodeless 2.75 MHz, 20 W lamp starting time can be 0.8-1.0 msec. Power consumption during starting interval of the ballast could be up to 80 W because of the high power losses in the lamp and the ballast. Therefore, the dimming method illustrated in FIG. 2 is not practical because of high power stresses applied to both lamp and ballast.
Other related art discloses a TRIAC dimmed electronic ballast that utilizes a charge pump concept for an inductively coupled lamp. But unfortunately, injecting RF power in front end 60 Hz power supply is not practical because of high-level EMI injected in the front end rectifier bridge. Accordingly, the 60 Hz rectifier bridge should be built with high frequency diodes for 2.5 MHz current. Another disadvantage of the concept is that, during lamp starting, a significant portion of RF transient power is taken from the ballast output to the charge pump. It may prevent the lamp from starting.
TRIAC dimmed electronic ballasts with a power charge pump feature a variable DC bus voltage resulting in lamp dimming. The charge pump requires high voltage bulk electrolytic capacitor connected to DC bus. Capacitor dimensions become a problem when the RF ballast is integrated in the lamp. Therefore, TRIAC dimming of high frequency ballast without electrolytic DC bus capacitor looks more attractive for RF ballast. But the restarting disadvantage mentioned above does not allow for practical implementation. Therefore, there is a need for other solutions for dimming high frequency electrodeless lamps.